This project concerns a novel strategy for the treatment of recurrent, therapy-resistant brain tumors with oncolytic immunotherapy. The oncolytic poliovirus recombinant, PVSRIPO, is in ongoing Phase-I clinical trials against recurrent glioblastoma (GBM). We observed promising complete clinical remissions with near-complete radiographic responses in several patients that failed prior surgery, chemo-, radiation-, and -in some cases - bevacizumab therapy. Remarkably, durable responses occurred with a single, ~6.5h intratumoral virus infusion with minimal adverse effects (none of which related to PVSRIPO). PVSRIPO is the live-attenuated poliovirus type 1 (Sabin) vaccine containing a foreign, rhinovirus type-2 internal ribosomal entry site (IRES). PVSRIPO is ?neuron-incompetent?, due to limits imposed by restrictive protein synthesis control in the normal CNS. Intracerebral infusion of up to the maximal feasible dose of PVSRIPO in humans confirmed the neuro-attenuated phenotype established in primate toxicology studies. Yet, PVSRIPO thrives in malignant cells ectopically expressing the polio receptor, the onco-fetal cell adhesion molecule Necl5/CD155. This is due to constitutively active MAPK signaling networks that favor an unorthodox mechanism of viral translation initiation. PVSRIPO infection of GBM cells has drastic cytolytic effects that produce sustained pro-inflammatory responses in animal tumor models. Lethal PVSRIPO infection of tumor cells and non-lethal transduction/ activation of antigen-presenting cells evokes complex immunologic reactions with the potential to reverse the inherently immune-suppressive tumor microenvironment and to elicit adaptive anti-tumor responses. This project is designed to investigate the principle of polio oncolytic virotherapy in the clinic and to unravel host immune responses to viral tumor infection. We propose three Aims: (1) Clinical investigation of PVSRIPO in patients with recurrent GBM. We will investigate the safety of PVSRIPO virotherapy and monitor the clinical and radiographic response in GBM patients; (2) Immune monitoring of PVSRIPO oncolytic immunotherapy in GBM patients. We are conducting a comprehensive immune monitoring effort to unravel host immune-mediated mechanisms of PVSRIPO oncolytic immunotherapy; (3) Elucidate mechanisms of poliovirus oncolytic virotherapy in a transgenic syngeneic mouse glioma model. We generated a novel, polio-susceptible, syngeneic tumor model that permits mechanistic investigations of PVSRIPO therapy in immune-competent hosts.